The prior art includes a "window shade" deployed space-fed phased array radar antenna which is particularly suited for use in space. The unrollable antenna is advantageous because it minimizes storage space aboard a spacecraft. When the spacecraft achieves selected orbit, the antenna is deployed and the "window shade" structure becomes actuated to a fully expanded operative condition. Such an antenna consists of a low-power RF feed which illuminates a lens aperture membrane. Active transmit/receive (T/R) modules in the aperture membrane receive radar pulses from the ground, amplify them, and perform beam-steering phase shifts so that the signal may be re-transmitted toward a target of interest in space. The reflected energy is received in reverse order, being amplified by the T/R modules then focused back onto the space feed. Radar processors and supporting subsystems are located in a bus at the base of a feed mast. A tensioned three-layer membrane constitutes the aperture and provides a very lightweight, yet sufficiently flat, aperture plane. Array flatness requirements for the space-fed approach are less severe than for corporate-fed approaches by an order of magnitude. The membrane aperture can be rolled up onto a drum resulting in a simple, compact, and repeatable method for deployment/retraction of the antenna.
An example of this type of antenna is shown in U.S. Pat. No. 4,771,817 to Angeloff, issued Sep. 20, 1988, to the present assignee.
In an effort to further increase the compact nature of this antenna, a drum arrangement exists which constitutes two separate pivotally connected drums which mount one side of two adjacent membranes. When stowed, the drums collapse against one another so as to reduce the necessary storage length by half. Upon deployment, the drums become arranged in coaxial adjacent fashion and mount one end of the deployed membranes. An opposite end of the membranes is secured to a collapsible end beam which, when deployed, rests parallel to the drum. Means are provided for sealing the seam between the deployed membranes in a shielded fashion. A means for sealing the adjacent antenna membrane edge is disclosed in U.S. Pat. No. 4,660,265 to Pallmeyer and issued Apr. 28, 1987, to the present assignee.
In an improved prior art embodiment shown in FIG. 1, the membrane is supported by two deployed coaxial drums 14A and 14B which are movably mounted to corresponding main beams 18A and 18B. In order to support an opposite end of the antenna membranes two end beams 30A and 30B become deployed. The inclusion of the end beams in addition to the main beams represents a weight and space problem which could be eliminated. Cable connections between the membranes and a bus located in the main beams must be routed to the ends of the main beam through rotary joints at the drum axles. The connections must then be routed back along the drums. This is a significant disadvantage since the cables carry relatively large amounts of DC power and RF signals which may be modified by connectors.
Further, DC power components have to be mounted inside the drums, which requires complicated mounting design and access as well as adequate achievement of thermal control.
The inboard ends of the drums must also be located quite close to each other, typically two inches. However, the inboard ends must also be securely fixed to the main beams. This presents a design dilemma due to the lack of room for structure in this space.